This invention relates to a method for non-destructive testing of components of gas turbine engines made of monocrystalline materials for the presence of cracks in a certain, critical component area upon expiry of a specific operating time, and in particular, for the inspection of turbine blades. Moreover, this invention relates to an apparatus for the performance of said method.
Non-destructive material testing using ultrasound for the detection of shrinkage, cracks and other flaws in the interior of a component has been known for quite a long time. The ultrasonic waves produced by the quartz oscillator of an ultrasonic probe positioned on the workpiece surface are transmitted into the component via a couplant. While—on a flawless component—the ultrasonic waves are reflected at the opposite bottom surface, return to the quartz oscillator now acting as receiver and produce a bottom echo, reflection at a discontinuity in the component results in a flaw signal which differs from the bottom echo. The shape of the flaw echo displayed on a screen enables the size, depth and type of the defect to be determined by comparing it with the bottom echo.
Inspection of components made of monocrystalline materials is, however, problematic since transverse ultrasonic waves are reflected differently on monocrystals, depending on the respective crystallographic orientation, as a result of which the signal reflected by a crack actually present in the component will not be reliably received and the discontinuity not safely detected.
In addition, ultrasonic inspection of monocrystalline components is difficult if the particular geometry of a component area to be flaw-tested leads to disturbance signals, as a result of which the reliability of the inspection is not ensured. Flaw detection is particularly problematic and costly if the components to be inspected are installed in a fixture and have to be removed for non-destructive testing and re-installed afterwards.
It is known of the blades of gas turbine engines, for example, that they may develop cracks in a certain area of the blade root. At certain intervals, it is therefore advisable to crack-inspect all turbine blades in question in a test laboratory by non-destructive methods. Apart from the high disassembly and assembly effort, the known methods and apparatuses are not capable of detecting, or excluding, crack formation in the interior of the blades in certain critical areas in a quick, safe and simple way. The sensitivity of X-rays used for this purpose is not sufficient to detect cracks in the interior of the turbine blades. Fluorescent inspection, as another well-known inspection method is, however, only suitable for the detection of superficial cracks.